Composite light source

ABSTRACT

Each of an array of &#39;&#39;&#39;&#39;n&#39;&#39;&#39;&#39; number of similar light sources is repetitively energized in sequence to stimulate emission of light energy therefrom into an optical system which sums the output of the individual devices and forms a composite output into a single beam having a frequency that is &#39;&#39;&#39;&#39;n&#39;&#39;&#39;&#39; times the frequency of each individual light source.

United States Patent Chow et al.

[451 Sept. 12, 1972 1 1 1 COMPOSITE LIGHT SOURCE [72] Inventors:Ken-Tang Chow, Portola Valley;

John William Stull, Livermore; Charles Edward Bates, Campbell,

all of Calif.

[73] Assignee: Electric-Nuclear Laboratories, Inc.,

Menlo Park, Calif.

[22] Filed: Oct. 10, 1969 211 App]. No.2 865,461

[52] US. Cl ..250/217 SS, 313/108 D, 331/945 [51] Int. Cl. ..G02f l/28[58] Field of Search ..250/84, 213 A, 217 SS, 220; 331/945; 313/108 D;307/311; 315/169;

[56] References Cited UNITED STATES PATENTS 3,310,753 3/1967 Burkhalter..331/94.5

3,311,844 3/1967 Di Curcio ..33 l/94.5 3,395,368 7/1968 Koester .,331/94.5 3,484,716 12/1969 Fenner ..33l/94.5 3,361,988 1/1968 Chynoweth..331/94.5 3,521,189 7/1970 Koenig ..331/94.5 3,396,344 8/1968 Broom..317/235 N 3,541,468 11/1970 Hammond ..331/94.5 3,590,248 6/1971Chatterton ..331/94.5

Primary Examiner-Walter Stolwein Assistant Examiner-D. C. NelmsAttorney-Eckhoff and Hoppe [57] ABSTRACT Each of an array of n number ofsimilar light sources is repetitively energized in sequence to stimulateemission of light energy therefrom into an optical system which sums theoutput of the individual devices and forms a composite output into asingle beam having a frequency that is 11" times the frequency of eachindividual light source.

10 Claims, 8 Drawing Figures PATENTED 3.691.390

SHEET 1 UF 5 INVENTORS KEN-TANG CHOW JOHN W. STULL BY WLES E. BATESATTORNEYS PATENTEDSEP 12 1912 SHEET 2 OF 5 2e 33 CLOCK I L.V.

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N v ll m T 52. 6 wzov mo jnaoz mm 3. Q a 9 K mU o M N m 88.2w N H. o N6% 0 85766 INVENTORS KEN-TANG CHOW BY JOHN w. STULL QCHAL E- 253 I HxooJo ATTORNEYS COMPOSITE LIGHT SOURCE This invention relates generallyto light-emitting devices and more particularly to a time-multiplexedarray of light-emitting devices which produces a composite radiantoutput having a frequency that is a multiple of the frequency of eachindividual device in the array.

The principle object of this invention is to provide a method and meansfor obtaining a composite light beam which has a frequency and powerthat is a multiple of the individual outputs of an array of periodicallyenergized light-emitting devices.

A particular object of this invention is to provide a method and meansfor obtaining a beam of light from an array of semiconductorlight-emitting devices at normal ambient temperature which is greater byseveral orders of magnitude in frequency and power than that heretoforeobtainable from one such device alone or connected in series or parallelwith similar devices.

A further object of this invention is to provide a method and means forobtaining such increase in frequency and power output, without thenecessity for external cooling, from two or more gallium arsenidecrystals periodically forward biased at current densities either aboveor below the threshold value for lasing action.

Still another object of this invention is to provide a method and meansfor producing a beam of coherent light from two or more gallium arsenidecrystals, without external cooling, which has a composite outputfrequency and power that are multiples of those parameters for eachindividual crystal.

Still another object of this invention is to provide a method and meansfor multiplexing an array of injection laser or spontaneouslight-emitting diodes.

Other objects and advantages of the invention will become apparent tothose skilled in the art upon consideration of the following descriptionand the accompanying drawings wherein FIG. 1 illustrates schematically aform of apparatus embodying this invention which is useful forpracticing its method with semiconductor light-emitting sources;

FIG. 2 is a sectional view of one of the individual semiconductor lightsources employed in the apparatus of FIG. 1;

FIG. 3 is a functional block diagram of the system of the invention withparticular reference to the embodiment of FIG. 1;

FIG. 4 plots the output pulse train of a single one of the semiconductorlight sources of the system of FIG.

FIG. 5 illustrates the output pulse train of the system of FIG. 3 using,for example, five semiconductor light sources;

FIG. 6 illustrates schematically a form of multiplexer employed in theembodiment shown in FIG. 3;

FIG. 7 is a diagram of the multiplexing time logic for the embodimentdescribed in FIG. 3; and

FIG. 8 is the circuit diagram of a specific embodiment of the inventionwhich employs five gallium arsenide laser diodes.

While the general method of and the apparatus components of theinvention are useful with a variety of light sources including tungstenincandescent lamps, neon lamps, mercury or xenon arc lamps, theinvention is particularly useful with semiconductor light-emittingdevices such as gallium arsenide diodes, and it overcomes a number oflimitations inherent in the use of such semiconductor devices as lightsources.

It has been known for some time that the passage of large currentsthrough a forward-biased semiconductor p-n junction diode will producelight radiation. It also is known that a gallium arsenide p-n junctiondiode when biased in the forward injection region by current densitiesgreater than a certain threshold value will emit radiation at roomtemperature corresponding to about 9,000 angstroms in wavelength whichcan be made coherent. At current densities below that threshold theradiation is an incoherent spontaneous emission.

The emitted radiation is coherent if l) the bias current density doesexceed the threshold value to produce an inverted population of energystates for lasing action and (2) the crystal, itself, is shaped into anoptical resonant cavity. The latter is done .by fabricating the two endsof the crystal parallelopiped very perpendicular to the plane of thejunction and polishing them to optical flatness. The index of refractionfor the air-gallium arsenide surface is high, so it is not necessary toreflectively coat the crystal ends. However, reflective coatings arefrequently used to lower the lasing threshold current value. Photonsproduced by applica tion of forward bias current in excess of thethreshold value travel along the path between the reflective ends of thecrystal and stimulate other electron-hole pairs to recombine to emit aphoton in phase with the stimulating photon. Reflection of the emittedphotons back and forth within the resonant cavity produces a standingWave. Since the crystal ends are only partially reflective, some of thatwave is emitted as a beam of coherent light along the plane of thejunction with a narrow spectral bandwidth.

Gallium arsenide diodes do, however, have very serious limitations whenoperated at current densities which will produce coherent lightemission. Recombination of electrons and holes in the lasing actioncreates instantaneous localized heating within the semiconductivematerial itself so that safe operating currents are generally no morethan about three times the threshold value. Moreover, other heatingeffects cause the lasing threshold current value to increase and theoutput radiant power at a given value above threshold to decrease with arise in temperature. Accordingly, a useful duty cycle for operationwithout external cooling and with a current of three times threshold,limits the output frequency for practical applications to about 4 KHz.

The present invention overcomes the limitations caused by these heatingeffects by combining the radiation output of an array of individualgallium arsenide diodes repetitively energized in sequence to obtain arelatively high power and high frequency output, for instance, asdescribed in the example in the order of 50 KHz.

The embodiment illustrated in FIG. 1 includes a plurality of injectionlaser diodes 10 arranged in an array directed to emit radiation into alight pipe 11 in which the radiation emitted from the several diodes ismixed and then transmitted to an optical lens system, designatedgenerally as 12, that forms the composite radiation into a beam with thedesired optical field.

Each injection laser diode 10, as is shown in FIG. 2, includes a galliumarsenide crystal 13 formed as a p-n junction. The crystal parallelopipedis an optical resonant cavity and has its two ends made veryperpendicular to the plane of the p-n junction and polished to opticalflatness so that radiation emitted from the crystal, as shown, isgenerally in the plane of FIG. 2. The crystal l3 mounts on anelectrically conductive base 14 within an envelope which includesnon-conductive cylindrical shell portion 15 and radiation (light)transparent portion or window 16 secured within the shell at the endopposite base 14. Electrode 17, integral with the base, connects oneelectrical contact of the crystal. Electrode l8, insulated from the baseby dielectric material 19, connects the other contact to supply biascurrent to the gallium arsenide p-n junction.

In the illustrated embodiment the light pipe 11 includes a largegenerally conically shaped collection portion 20 which gathers radiationfrom the plurality of injection laser diodes 10 mounted at its large endso that they emit radiation through their respective windows generallyalong the longitudinal axis of the light pipe. A cylindrical portion 21of the light pipe at the small end of the collection portion 20transmits the collective radiation from the several diodes to an opticallens shaping system 12. The light pipe emits the collective radiationthrough output aperture 22 to the lens system.

The light pipe is solid light transmissive material such as quartz ormolded plastic. Except for aperture 22, a highly diffuse reflectivecoating 23 of magnesium oxide or titanium oxide coats all exteriorsurfaces of the light pipe 1 1, including the conical collection portion20 and the cylindrical portion 21. Light energy radiated into the pipefrom the several injection laser diodes by repeated reflection from thereflectively coated side walls of the light pipe in passing through thepipe is thoroughly mixed and scrambled before it emerges from thenon-coated aperture 22.

FIG. 3 illustrates schematically one form of circuit whereby a pluralityof injection laser diodes 10 in an array are time-multiplexed so thattheir combined output at aperture 22 has a frequency n times thefrequency of that of a single diode, where n is the number of diodes inthe array. The light output from each diode, designated in FIG. 3 as10a, 10b, 10c, 10d 10n, radiates into the light pipe 11 and the outputaperture 22 emits the combined radiation to the optical lens system 12of FIG 1.

The system of FIG. 3 repetitively supplies bias current to each of theinjection laser diodes in sequence in short pulses in excess of thethreshold value for lasing action in the order of 50 nanosecondsduration, for example. FIG. 4 illustrates the resultant output power fora single one of the diodes. FIG. illustrates the output power for thesystem of FIG. 3 assuming n is a total of five diodes, A through E,pulsed in sequence with bias current having the same 50 nanosecond pulsewidth but separated in time by 250 microseconds. The collective outputof the array of diodes thus is the sum of the output of each diode whenthis output is gathered and optically guided in light pipe 11 to emitover the same optical field. The collective output frequency is amultiple of the number of pulsed diodes in the array or in the example,5 times the frequency of a single diode. It will be apparent that thistechnique can be used to achieve practically any output frequency byselection of the number of diodes.

A master clock 25 operating at the output pulse repetition rate of thesystem, f,, triggers a binary or modulo-n" counter 26 wherein n is thenumber of laser diodes to be multiplexed. A l/n decoder 27 detects eachstate of the counter [0-(nl or its minterms and produces a pulse on oneof the n output lines of the decoder. FIG. 6 is a more detailed diagramof this part of the system. The modulo-n counter or binary counter 26 ismade up of in number of bi-stable multivibrators 28, where m equals logn. The counter outputSfo/2,fl,/4,f,,/8 f,,/2" are fed into the decoder27 which may be a diode matrix or series of logic gates that produces aunique output on one of its n output lines so that only one is active atany given time.

The time relationship between the clock pulses f,,, the frequencydivision produced by the binary counter outputs f l2, f,,/4, etc., andthe sequence of min-term outputs 1 through 11 of the decoder appear onFIG. 7.

Each of the repetitive output pulses on the n" output lines of decoder27 operates a corresponding trigger circuit 29a, 29b, 29c, 29d, 29n.Each trigger circuit gates on a corresponding silicon controlledrectifier switch 30a, 30b, 30c, 30d, 30n. Each switch connects one ofthe array of laser diodes 10a, 10b, etc. to a pulse forming network 31that supplies forward bias current at a level in excess of the thresholdvalue for lasing action from high voltage supply 32. Energy foroperating each of the laser diodes is stored in the pulse formingnetwork, the impedance of which matches that of the diode in series witha small ballast resistor. The network determines the pulse width and thevoltage to which the network is charged by supply 32 determines theamplitude of the pulses.

Power supply 33 drives the binary counter 26, decoder 27 and the severaltrigger circuits 29a, 29b, etc.

The described system thus produces for an array of injection laserdiodes, a composite output radiation which has a frequency that is amultiple (by the number of diodes in the'array) of the frequency of eachindividual diode and an average power which is the same multiple at roomtemperature without cooling.

The system has been configured to produce at ambient room temperaturesan output frequency of KHz using five gallium arsenide laser diodesoperated in the circuit shown in FIG. 8. Clock 25 is a multivibratorwhich supplies pulses at repetition rate f, to trigger modulo-5 counter26. It is a Signetics 8280J decade counter connected in the bi-quinarymode. Decoder 27 is an array of logic gates that produces a uniqueoutput on one of its five output lines in response to the state of thecounter outputs supplied to it. In this embodiment decoder 27 is aSignetics 8251 BCD-to-decimal decoder. The repetitive output pulses onthe five decoder output lines pass to the series of two input positiveNAND gates 29 here shown as a pair of Texas Instruments Type SN 7400 hexinverter microcircuit elements. Each gate output switches on a siliconcontrolled rectifier switch, 30d for example, in one of five similarmodulator circuits incorporating the laser diodes, 10d for example, andpulse forming network elements.

The foregoing specific embodiment is described for illustrative purposesonly. It will be apparent to those skilled in this art thatmodifications to the structure may be practiced and equivalentssubstituted for the specific elements described which are within thescope of the invention defined in the following claims.

We claim:

1. A method for multiplying the frequency of the output from aperiodically electrically energized light source comprising 4 forming anarray of a plurality of similar closely adjacent light sources;

repetitively electrically energizing in sequence each of the lightsources in said array;

gathering and optically mixing the light emitted from the severalenergized light sources; and

then optically guiding said light into a composite beam having a comm'onoptica] field for the light emitted from each light source.

2. The method of claim 1 wherein said light sources are semiconductorlight-emitting devices.

3. The method of claim 1 wherein said light sources are forward-biasedgallium arsenide p-n junction diodes.

4. The method of claim 1 wherein said light sources are forward-biasedgallium arsenide p-n junction diodes periodically energized with biascurrent in excess of the threshold density requisite for lasing action.

5. Apparatus for multiplying the frequency of the output from aperiodically electrically energized light source comprising an array ofsimilar light sources in close spatial relationship;

means for repetitively electrically energizing in sequence each of thelight sources in said array;

optical means for gathering and mixing the light emitted from theseveral energized light sources; and

optical means guiding said light into a composite beam having a commonoptical field for the light emitted from each light source.

6. The apparatus of claim 5 wherein said light sources are semiconductorlight-emitting devices.

7. The apparatus of claim 5 wherein said light sources areforward-biased gallium arsenide p-n junction diodes.

8. The apparatus of claim 5 wherein said light sources areforward-biased gallium arsenide p-n junction diodes and wherein saidmeans for energizing the light sources supplies bias current to saiddiodes in excess of the threshold density requisite for lasing action.

9. The apparatus of claim 5 wherein said means for energizing said lightsources includes a supply of bias current, gate means for connectingsaid supply to one of said light sources at a time, and clock controlledmeans for enabling said gate means to repetitively connect each lightsource in sequence to said supply.

10. The apparatus of claim 5 wherein said optical means for gatheringand mixing light from the light sources comprises a light transmissiveconduit, having a large end gathering radiation emitted from said lightsources and a smaller end for emitting the combined light from all ofsaid sources, an optically reflective coating on the exterior surface ofsaid conduit, and a light transparent aperture at the small end of saidcon- 7 Patent No. 3,691,390

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated September 12I972 Ken-Tang Chow; John William Stull; Charles Edward Bates;

It is certified that error appeairs in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

On cover sheet, change assignee to read:

Eleccg-Nuclear Laboratories, Inc.

Signed and sealed this 11th day of February 1975'.

(SEAL) Attest:

RUTH c. MASON Attesting Officer C. MARSHALL DANN Comissioner of Patentsand Trademarks ORM PO-IOSO (10-69) wUSCOMM-DC 60376-P69 i U.S.GOVERNMENT PRINTING OFFICE: I969 0-356-33L

1. A method for multiplying the frequency of the output from aperiodically electrically energized light source comprising forming anarray of a plurality of similar closely adjacent light sources;repetitively electrically energizing in sequence each of the lightsources in said array; gathering and optically mixing the light emittedfrom the several energized light sources; and then optically guidingsaid light into a composite beam having a common optical field for thelight emitted from each light source.
 2. The method of claim 1 whereinsaid light sources are semiconductor light-emittIng devices.
 3. Themethod of claim 1 wherein said light sources are forward-biased galliumarsenide p-n junction diodes.
 4. The method of claim 1 wherein saidlight sources are forward-biased gallium arsenide p-n junction diodesperiodically energized with bias current in excess of the thresholddensity requisite for lasing action.
 5. Apparatus for multiplying thefrequency of the output from a periodically electrically energized lightsource comprising an array of similar light sources in close spatialrelationship; means for repetitively electrically energizing in sequenceeach of the light sources in said array; optical means for gathering andmixing the light emitted from the several energized light sources; andoptical means guiding said light into a composite beam having a commonoptical field for the light emitted from each light source.
 6. Theapparatus of claim 5 wherein said light sources are semiconductorlight-emitting devices.
 7. The apparatus of claim 5 wherein said lightsources are forward-biased gallium arsenide p-n junction diodes.
 8. Theapparatus of claim 5 wherein said light sources are forward-biasedgallium arsenide p-n junction diodes and wherein said means forenergizing the light sources supplies bias current to said diodes inexcess of the threshold density requisite for lasing action.
 9. Theapparatus of claim 5 wherein said means for energizing said lightsources includes a supply of bias current, gate means for connectingsaid supply to one of said light sources at a time, and clock controlledmeans for enabling said gate means to repetitively connect each lightsource in sequence to said supply.
 10. The apparatus of claim 5 whereinsaid optical means for gathering and mixing light from the light sourcescomprises a light transmissive conduit, having a large end gatheringradiation emitted from said light sources and a smaller end for emittingthe combined light from all of said sources, an optically reflectivecoating on the exterior surface of said conduit, and a light transparentaperture at the small end of said conduit.